Licensed spectra, that is, wireless spectra that operators are authorized to use as regulated by the laws of various countries, can be used only by those having a license and authorization and having passed the network entry test. Unlicensed spectra, for example, working frequency bands of WiFi and Bluetooth devices, can be used without authorization from the government. Because the shortage of licensed spectrum resources allocated to mobile communication restricts the development scale of mobile communications networks in the future, combining licensed spectra with the large number of unlicensed spectra to improve the transmission capacity of mobile networks is currently being considered. Statistics indicate that currently in the 5 GHz frequency band, unlicensed spectrum resources that have been allocated in various countries may exceed 500 MHz in total. These unlicensed WiFi spectra have attracted attention from mobile communications equipment vendors, mobile network operators and so on. Accordingly, solutions such as LTE-U (Unlicensed LTE, Unlicensed LTE) emerge, and the launch of projects for testing the Unlicensed LTE technology was recently announced. The license assisted access (License Assisted Access, LAA) technology that the international standardization organization 3GPP is currently studying is a typical solution of expanding the capacity of mobile communications systems by using unlicensed spectra, which allows an LTE system to use unlicensed spectra to further improve the transmission capacity of the LTE system.
In an LAA system, a base station may use an unlicensed spectrum to increase the user throughput and the system capacity. However, because multiple systems share a same unlicensed spectrum, a conventional contention mechanism is generally adopted to determine how to use the unlicensed spectrum, in order to enable the systems to have the same opportunity to access the unlicensed spectrum. Using uplink scheduling of the LAA system as an example, for example, a mainstream implementation method that is proposed at present is as follows: A base station first schedules some user equipments (User Equipment, UE) at a scheduling moment, and determines, in a scheduling instruction (UL Grant), information such as a time-frequency resource occupied by a user and a transmission format, to instruct the user equipments to prepare for uplink transmission. After several frames, the base station performs clear channel assessment (Clear Channel Assessment, CCA) to determine whether a current channel is in a busy or idle state, and when the channel is idle, sends a physical layer signal (for example, a reservation signal (Reservation Signal)) to occupy the channel. At a next transmission moment, the user equipment sends uplink data according to the time-frequency resource and the transmission format that are indicated by the base station. It can be seen that different from a conventional mobile communications system using a licensed spectrum, the LAA base station cannot determine, at the scheduling moment, whether an unlicensed carrier is available at the transmission moment after several frames that is indicated by the scheduling instruction, that is, whether the unlicensed spectrum is available is unpredictable.
As shown in FIG. 1, the base station schedules a user equipment (User Equipment, UE) by using a scheduling instruction (UL Grant) in the nth subframe, wherein it is indicated in the UL Grant that a carrier resource allocated to the user equipment is a carrier 2, and it is also determined in the UL Grant that information such as a format used for transmission, to instruct the user equipment to prepare for uplink transmission. In the (n+3)th subframe, the base station performs clear channel assessment to determine whether a current channel is in a busy or idle state, preempts the idle carrier 2, and sends a Reservation Signal on the carrier 2 to occupy the channel. Then, in the (n+4)th subframe, the user equipment sends uplink data on the carrier 2 according to the transmission format indicated by the base station. However, if the base station fails to preempt the carrier 2, there will be no available transmission resource for the user equipment at the transmission moment; or if the base station has preempted a carrier 3 but does not allocate the carrier 3 to the user equipment at the scheduling moment, the resource is wasted.